user clip plane

anyone could help me? thanks a lot!!!!!!!!

It is not clear. Three planes could divide the frustum in different ways. Maybe you are talking on dividing the client area of the window... in this case, you could use viewports.
Give us more details.

Hi, thanks.
these planes parallel to view plane, and then render each of them to a texutre from the light's position. how can I do that?

Yes, these planes divide the frustum in different z values, and then I wanna render each of these partitioned part into a texture. and such kind of rendering into texture isn't from the viewpoint, but another position.
u should know "Shadow Mapping", first we render the scene into a texture from the light's position in this technique. that's same as what I wanna do.